This invention generally relates to electrical machines, and more particularly, to means for cooling the rotors in both motors and generators during operation.
In a large superconducting synchronous generator the reliable operation of the superconducting field winding at high current densities is critical. In these machines high current densities in the field winding can be achieved only when the temperature of the winding is constantly maintained at or below the boiling point of liquid helium. In a typical filamentary niobium-titanium superconductor, at magnetic fields of 7.5 Tesla, each degree of temperature drop of below 4.2.degree. K. increases the critical current density of the superconductor by approximately 50%.
One type of superconducting generator is illustrated in FIG. 1. The generator includes a rotor 6 that is supported by two bearings 10 in a housing 7. The rotor turns within the stator bars 8 and is rotated by a turbine (not shown) attached to the coupling 12. The field winding in the rotor is energized through the collector rings 14.
The rotor 6 contains a superconducting field winding 16 which is cooled to liquid helium operating temperatures. The winding is housed within an electromagnetic shield 18 that also serves as a vacuum envelope for the winding. The interior of the rotor at points 19 is permanently evacuated in order to insulate the rotor from other components of the generator operating at ambient temperature. The electromagnetic shield also screens the superconducting winding from non-sychronous components of the magnetic fields produced by unbalanced or transient currents in the stator 8. Inside of the electromagnetic shield is a thermal radiation shield 20 which is cooled to an intermediate temperature of between 80.degree. K. and 100.degree. K. This shield absorbs thermal radiation from the ambient temperature electromagnetic shield 18 and re-radiates it at a lower temperature. The winding 16 is also protected at its ends by two thermal radiation shields 21, 21'.
Inside of these shields is the torque tube 23 which transmits the torsional forces from the rotor winding 16 through the coupling 12 to the turbine (not shown). The torque tube is illustrated in FIG. 2 in end elevation. The torque tube houses nine, epoxy resin impregnated, superconducting winding modules 16. Each superconducting winding module is racetrack shaped and is manufactured from stabilized, niobium-titanium multifilamentary superconductor. The superconductor is wound and impregnated with epoxy resin. The epoxy resin is reinforced with glass cloth. Referring to FIG. 3, each winding module 16 is supported by a thick aluminum housing 26 that is machined from a rectangular plate to fit the outer surface of each winding module. Referring to FIGS. 2 and 5, spaced between adjacent housings 26 is a thin aluminum support plate 28. The longitudinal sides of each plate are machined to form wedges that fit the tapered edges of the corresponding housings 26. In addition, two aluminum pole segments 30, FIG. 2, are located at opposite ends of the winding stack. The assembly of winding modules 16, housings 26, support plates 28, and pole segments 30 is fastened together by five cross bolts 32 located along the longitudinal axis of the rotor. These cross bolts hold the structure together so that it can be inserted into the cylindrical torque tube 23, FIG. 2. The torque tube is shrunk onto the assembly, and the assembly is prevented from rotating relative to the torque tube by a plurality of keys (not shown). The torque tube is fabricated from either a FeNiCr-base austenitic stainless steel or a nickel-base stainless steel.
Referring to FIG. 1, the field winding 16 is cooled by the flow of liquid helium through the rotor. Saturated liquid helium is delivered to a central supply tube 36 from a liquefier or supply dewar (not shown). The liquid helium flows along the axis of rotation of the rotor into the torque tube 23. The liquid helium is distributed in the rotor by a radial supply tube 38 and a level control 39.
In steady-state operation the liquid helium boils as a result of the heat transferred into the cold region of the rotor. Two separate streams of boil-off vapor are removed from the rotor. One stream passes through a series of spiral flow channels 41, then through passage 42 that runs across the inner side wall of the electromagnetic shield 6 and thereafter through the exhaust tube 43 which is concentric with the central supply tube 36. The other stream of vapor passes through a second plurality of spiral flow channels 41' which also connect to the concentric exhaust tube 43. The warm helium vapor thereafter flows out of the generator and is returned to the liquefier (not shown).
When the rotor 6 turns, the centrifugal force on the rotor causes the liquid to assume the shape of a cylinder having an annular cross section, FIG. 2. The lighter weight helium vapor becomes centered about the axis of rotation of the rotor. The cylindrically shaped surface between the liquid and vapor is indicated by reference numeral 46, FIG. 2. The pool of liquid helium rotates at the peripheral speed of the rotor. The level control 39, FIG. 1, regulates the radial depth of the liquid. The pressure of the helium vapor at the liquid pool surface 46 is maintained at a predetermined value by the balance of the hydrostatic pressures in the rotating gravitational field. The maintenance of the pressure of the vapor at a predetermined value fixes the temperature of the liquid helium at the surface 46. This temperature corresponds to the saturation temperature of helium at the specified pressure.
One problem with helium cooled rotors of the type described above is minimizing the radial temperature gradient created across the cylindrical pool of liquid helium. When the pool of liquid helium rotates with the rotor and thermal conduction in the radial direction is low, a temperature gradient is established within the liquid. The temperature distribution within the pool corresponds to the adiabatic temperature rise of a particle of fluid as it moves radially within the pool and is compressed by the local equilibrium hydrostatic pressure. The pool is stable until the temperature gradient exceeds the gradient corresponding to adiabatic compression. At this point bouyancy forces will cause circulation of the warmer, less dense fluid to the surface of the pool, where it is cooled by boiling or evaporation. Each superconducting winding module 16 operates at some intermediate temperature between the temperature of the fluid at the interface 46 and the outer radius 34, FIG. 2, of the liquid cylinder.
Another problem occurs if the temperature difference between the superconductor and the coldest portion of the liquid helium cannot be substantially reduced. The temperature of the superconductor in the winding is nearly uniform. Currently, these windings are covered with a layer of glass reinforced epoxy resin which is the major thermal resistance to transferring heat from the winding to the liquid helium. The thermal conductivity of the area filled with superconductor is relatively high. The problem arises because the most critical location where quench is initiated is along the inner radius of the winding and insufficient heat transfer causes the temperature of the winding near the inner radius to increase to an intermediate value between the temperatures at the inner and the outer pool radii.
Further, recent studies have shown that substantially more thermal margin is needed over prior methods of cooling in order to maintain superconductivity in the windings during accidental temperature excursions. For example, if the high voltage side of a generator transformer is short-circuited, the heat input to the rotor from the circulating currents generated in the torque tube is substantial. This heat input causes a rise in the temperature of the liquid helium in the vicinity of the torque tube, thereby causing the superconducting winding temperature to rise. Prior winding support structures are incapable of transferring the heat generated by the electrical transient to the liquid helium without excessively heating the windings.
Heretofore, solutions to these problems have included adding passages for liquid helium in close proximity to the winding 16 and machining the winding support structure 28 for optimum thermal contact with the winding modules 16. The support structure 28 has also been fabricated from aluminum so that it can act as a thermal fin. A shallow liquid helium pool has also been used to minimize the temperature difference caused by compression of the liquid in the pool.
Although all of these approaches have proven to have benefit during steady-state operation, the problem of minimizing the heat input into the superconducting windings during electrical transients, and effectively removing the heat generated in the torque tube has never been effectively solved.